1. Field of the Invention
The present invention relates in general to a control apparatus for a vehicular power transmitting system including an automatic transmission portion, and more particularly to techniques for improving a control response of shifting actions of the automatic transmission portion.
2. Discussion of Prior Art
There is known a power transmitting system suitable for hybrid vehicle, including an electrically controlled differential portion which has a first electric motor and a differential mechanism a differential state of which is controllable by controlling an operating state of the first electric motor, and further including a step-variable automatic transmission portion constituting a part of a power transmitting path, and a second electric motor connected to the power transmitting path. JP-2005-264762 A discloses an example of such a vehicular power transmitting system. A control apparatus for the vehicular power transmitting system disclosed in this publication is configured such that a rise of the rotating speed of an output shaft of the electrically controlled differential portion causes a rise of the operating speed of the first electric motor in the negative direction during a shift-down action of the automatic transmission portion, as is apparent from a collinear chart of the FIG. 3 in the above-identified publication. This phenomenon increases with a decrease of the operating speed of an engine. To prevent an excessive rise of the rotating speed of the first electric motor, there is provided a lower limit of the operating speed of the engine so that a shift-down action of the automatic transmission portion according to a shifting command during running of a vehicle by the engine in an engine-drive mode is inhibited until the operating speed of the engine which is lower than the above-indicated lower limit upon generation of the shifting command is raised to the lower limit. However, this aspect of the control apparatus is not described in the publication. When an accelerator pedal of the vehicle is depressed by a considerably large amount by an operator of the vehicle, the control apparatus may generate a shifting command to perform a jump or skipping shift-down action from the present gear position to a target gear position while skipping at least one intermediate gear position between the present gear position and the target gear position. Where this jump shift-down action according to the shifting command is inhibited due to the lower limit of the engine speed, the automatic transmission portion is controlled to perform a sequential or stepping shift-down operation wherein the automatic transmission portion is sequentially shifted from the present gear position to each of the at least one intermediate gear position, and from the lowest intermediate gear position to the target gear position.
The control apparatus for the vehicular power transmitting system disclosed in the above-identified publication permits the automatic transmission portion to be shifted down to the target gear position according to the shifting command to perform the jump shift-down action, while preventing an excessive rise of the operating speed of the first electric motor. It is noted that the operating speed of the engine is usually in the process of rising under a situation in which the jump shift-down command to perform the jump shift-down action is generated. In view of this tendency, it is considered that the automatic transmission portion can be shifted down to the target gear position at an earlier point of time where the jump shift-down action is performed after the engine speed is raised to the lower limit, than where the sequential shift-down operation is performed, particularly when the rate of rise of the engine speed is relatively high. This possibility is not described in the above-identified publication. Namely, the control apparatus disclosed in the publication is not configured to assure a high degree of control response of the jump shift-down actions under all situations in which the jump shift-down commands are generated.